Nocturnal enuresis, or nightly bedwetting in children more than seven years of age affects about 10% of seven-year-old children, with a wide range of frequencies between populations. The affliction is often linked to major social maladjustments and occupies considerable time in general practice. From the age of seven there is a spontaneous cure rate of 15% per year, such that few remain affected after the age of 16 years. There are two types of nocturnal enuresis: type I (PEN1, primary) with at least three nightly episodes in children above seven years, where the child has always had the disorder and type II (secondary) where the child has been dry for at least six months, but enuresis has recurred. Among some 400 Danish, mostly three-generation families, we have found 17 families with nocturnal enuresis. Eleven of these family had type I nocturnal enuresis (PEN1) that appeared to follow an autosomal dominant mode of inheritance with penetrance above 90%. We now describe strong evidence of linkage with the DNA polymorphisms D13S291 (Z = 3.55; theta M = F = 0.07) and D13S263 (Z = 2.67; theta M = F = 0.08). Multipoint analysis indicates that these markers flank the disease locus at chromosome 13q13-q14.3.